Does she have standing BHS? If not, start with that. On tumbletrak, then on a reasonably-springy mat (a newish 4- or 8-incher works best, but you can also take an older softer mat and unfold panel mats on top of it), then on the floor.
You want her to start and finish in exactly the same position, what I call "curled" or "question mark" position; back rounded, hips mostly tucked under, knees slightly bent, eyes on the toes, arms down, wrists curled so fingertips are right in front of the knees. She should start from this position with no additional arm swing, and finish by sticking (NO REBOUND!) in exactly this same position.
Once she can do a standing BHS that starts and ends in this position, you're done spotting her on backhandsprings forever. You should never need to spot another one after this point.
Next step: on whatever surface she's comfortable, she does this standing BHS to and from curled position and sticks with no rebound whatsoever, and no movement of the feet after landing. Once she lands, she stops however long she needs to in this position (NO MOVING THE FEET OR SWINGING THE ARMS), then does another standing BHS from that landing position. Since by this point she can both land an takeoff from this same position, she can string any arbitrary number of BHS together in this manner: curl, standing BHS to curl, stop as long as she likes, repeat. I like the athlete to be able to do at least three like this, but your only real limitation is the length of your tumbletrak/strip of floor/whatever.
Have her do this a million times, and magic will happen. Don't tell her to connect, don't tell her not to connect, just tell her to do consecutive BHS that start and finish in curl; over time, the pause in between will get shorter and shorter as she gains confidence, until it disappears completely, and she's doing connected series of standing BHS.
Take this from tumbletrak to springy mats to floor.
Then replace the first BHS in the series with a knee-lunge roundoff. Then a fall-step roundoff. Then one-step hurdle roundoff.
This approach has four major advantages:
1) By stopping in curled shape in each connection, you emphasize correct connecting technique with good shapes and a properly-patient sit-back in between each skill
2) The spotting required is minimal, even when adding multiple BHS in series
3) Because they know they can stop in between skills, they feel a level of control that tends to make mental blocks less likely
4) Should she develop a back tumbling mental block down the road, she's got easy steps she can go back to and work her way back up
EDIT to add: I started teaching RO-BHS with this method about 12 years ago. Among the kids that have gone through this process with me in that time, not a single one has developed undercut backhandsprings, and not a single one has developed significant back tumbling mental blocks.
